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Sugimoto string theory

The Sugimoto string theory is a non-supersymmetric version of type I string theory, given by 10d type IIB string theory on an $O9^+$ orientifold (instead of $O9^-$), hence with RR-field tadpole cancellation by 32 anti D9-branes (instead of plain D9-branes), whose gauge group is the symplectic group $USp(32)$.

References

The original article:

• Shigeki Sugimoto: Anomaly Cancellations in the Type I D9-anti-D9 System and the $USp(32)$ String Theory, Prog. Theor. Phys. 102 (1999) 685-699 [arXiv:hep-th/9905159, doi:10.1143/PTP.102.685]

Further discussion:

• Sanefumi Moriyama: $USp(32)$ String as Spontaneously Supersymmetry Broken Theory, Phys. Lett. B 522 (2001) 177-180 [arXiv:hep-th/0107203, doi:10.1016/S0370-2693(01)01278-3]

• Hector Parra de Freitas, p. 166 of: String Compactifications with Half-maximal Supersymmetry, PhD thesis, Université Paris-Saclay (2023) [hal/tel-04234221, pdf]